This invention relates to an improvement of the drum brake for use in an automotive vehicle. More particularly, it is aimed at improving the construction of a substantially U-shaped return spring, which is disposed between a pair of brake shoes (hereinafter simply called shoes); the spring draws the shoes toward one another in a drum brake for vehicle use, thereby preventing the shoes from interfering with the backing plate, the parking lever, or other component of the brake, by means of a definite positioning of the return spring after the same has been mounted on the shoe.
In a drum brake the braking function is carried out by making a pair of shoes slidingly contact a rotating drum which rotates with a wheel. Each shoe is made of a crescent-shaped member with a lining thereon. A wheel cylinder, disposed between the extreme end portions of the pair of shoes, actuates the contact between the shoe and the drum. It is necessary that the shoe, which has been slidingly urged on the drum, be retracted therefrom rapidly and positively as soon as the actuating force on the shoe disappears. For attaining this object a return spring is tensed and anchored between the pair of shoes, whereby the latter are biased, by means of elasticity of the return spring, in a withdrawing direction from the contacting surface of the drum, i.e., in a mutually approaching direction. As a return spring for this purpose a coil spring or a substantially U-shaped spring made of a rigid wire rod is generally used. In case of small sized drum brakes, however, a U-shaped return spring is preferable to a coil spring, because of the former's considerably freer choice of positioning without being interfered with or being affected by other brake component members.
In such a conventional U-shaped return spring, each L-shaped end portion thereof is engaged with a respective engaging hole which is formed on either shoe web. The extreme end of both bent portions are bent or curved in a hook-like manner, still further, for preventing the bent portion, which has been engaged with the engaging hole, from slipping out of the hole.
Such a construction of the bent portion with a hook-like curved end is solely aimed at preventing the spring from slipping out of the engaging hole, and no regard is paid to the problem of whether the bent portion comes into the engaging hole of the shoe web so deeply as to protrude too great a distance toward the backing plate side, when some kind of urging force is actuated on the return spring. This phenomenon could cause the protruding portion of the return spring, to interfere with the backing plate and/or the parking lever, thereby producing an inferior braking effect because of an undesirable sliding of the shoe or a deteriorated operation of the parking lever.
A similarly constructed U-shaped return spring to the above, disclosed in U.S. Pat. No. 3,441,109, is also one on which the end portions thereof are bent, with the object of engaging the curved portion, formed by bending, directly with the engaging hole of the web. It still contains various knotty problems such as:
(1) Because of technical difficulty in forming a curved portion with a small radius of curvature at the furthest end of the spring a surplus length of wire remains, on each extreme end portion of the spring. This surplus length of wire must be cut after the bending process has been finished. The choice of the cut position is not an easy matter, because the cut position must be as close as possible to the curved portion in order to minimize the likelihood that the curved portion will protrude from the engaging hole toward the backing plate side, thereby preventing the same from interfering with other brake component members, such as the backing plate. The cut position must also be as far as possible from the curved portion with a sufficient allowance left for securing the engagement of the bent portion with the engaging hole.
(2) A further serious problem is the likelihood that a crack or cracks will appear on the outer side of the curved portion during the bending process, which portion is required to be a hook-like portion with a small radius of curvature for securely engaging with the hole. Such a crack or cracks must be absolutely avoided for fear of damaging the spring.
(3) The problem of minimizing the cost of cutting off a piece of material from the spring end is not a negligible one.